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Transcript of 01 Introduction.pdf
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The Interstellar
Medium
Lecturer: Dr. Paul van der Werf
Oortgebouw 565, ext 5883
Assistant: Kirstin Doney
Huygenslaboratorium 528
http://www.strw.leidenuniv.nl/~pvdwerf/teaching/ISM/
Fall 2014
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Physical basis (microphysics)
Discussion of the composition and physical processes of the ISM in
order of increasing complexity
Develop physical understanding of physics, diagnostics, and life
cycle of ISM
Very active field of research with key links to stellar evolution,
galactic structure, & galaxy evolution
Aims
Overview of the structure and constituents of the ISM
Understanding of the physical processes shaping the ISM
Appreciation of the role of the ISM in the evolution of galaxies
Methods
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Required background
Radiative processes (Planck function, Einstein coefficients, ...)
Statistical physics (Boltzmann and Maxwell distributions)
Quantum physics (H-atom)
Required literature
Draine, Physics of the Interstellar and Intergalactic Medium
(Princeton University Press) - REQUIRED
Book emphasizes theoretical foundations observational aspects and
practical cases will be given in class
Other (non-required) literature listed on class website
Slides will be available from course website after the lecture
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Class OrganizationLectures most Fridays (see class website) 11:1513:00, HL414
Problem classes on 5 Friday afternoons (see class website)
Problem sets must be handed in. Average of problem sets counts for
25% of final grade. They must be done in groups of 2 people (one
group of 3 if necessary) because of their challenging nature and in
order to enforce discussion.
Exam: oral, by appointment, in December 2014 or January 2015
Study tipsRead the relevant book sections before the lecture
During the lecture, make notes
After the lecture, study the relevant book sections; see class website
for details
Problem sets are (very) challenging and time-consuming! Start on
time!
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Class Schedule (full details on website)
1. Introduction. Basic physical processes Draine Ch 1
2. Emission and absorption processes. Radiative transfer Draine Ch 6 & 7
3. The HI 21cm line Draine Ch 8 & 9
4. Ionization and recombination Draine Ch 12, 13 & parts of Ch 14
5. Photoionization and HII regions Draine Ch 15 (parts)
6. Collisional excitation. Nebular diagnostics Draine Ch 17 & parts of Ch 18
7. Molecular energy levels and excitation. Radiative trapping Draine Ch 5 (parts) & 19
8. Interstellar dust Draine Ch 21 & parts of 23 & 24
9. Thermal balance and the two-phase model of the ISM Draine Ch 27 (parts) & 28, 29, 30
10. Molecular clouds Draine Ch 31 (parts) & 32
11. Shocks, supernova remnants and the 3-phase ISM model Parts of Draine Ch 35, 36 & 39
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Material (almost) not covered
Astrochemistry (see lectures Ewine van Dishoeck)
Star formation (see lectures Ewine van Dishoeck)
Collisionally ionized nebulae (novae, supernovae)
Optical properties of dust grains
Astrophysical gas dynamics
Radio continuum emission (see Radiative Processes course)
Masers
Magnetic fields
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Todays Lecture
Class introduction
ISM: history of discovery and research
Constituents of the ISM
Basic physical conditions
In red: essential exam material
Corresponding textbook material: Draine Ch. 1
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History of ISM research
1656: Huygens describes the Orion Nebula
~1800: W. Herschel, catalog of bright patches called nebulae
1864: Huggins, spectra of Andromeda (Sun-like) and Orion (gaseous emission) nebulae
1904: Hartmann, stationary Ca II lines in spectrum of spectroscopic binary Ori
Discovery of ISM
1919: Barnard, catalog of dark nebulae holes in stellar distribution or obscuring matter?
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Huygens describes the
Orion Nebula (1656)
Huygens, Systema Saturnium
(1656)
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NGC 1976: the Orion Nebula
HST image
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Dark cloud B68
ESO-VLT
Alves et al. 2001
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History (contd)
1937 40: Swings & Rosenfeld, McKellar, Adams, first
small interstellar molecules (CH, CH+, CN)
Spectrum toward by Adams (1941), showing the sharp
interstellar CH and CN lines superposed on the broad stellar He line
CN CH
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History (contd)
1945: prediction of HI 21cm line by Van de Hulst
1951: Ewen & Purcell, Oort & Muller, detection of 21 cm
line
1950s 60s: 21 cm maps galactic disk contains 5x109
M of gas (10% of disk mass)
=1 cm-3
1968: NH3 (first polyatomic molecule)
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History (contd)
1970: CO J = 10 emission at 2.6 mm
1970s-1980s: Galactic distribution of CO
Distribution molecular vs atomic gas
1970s - now: Many new interstellar molecules found (>100); some very exotic
1973: Carruthers, UV lines of H2 from rocket
1970s 80s: Infrared astronomy (H2 infrared lines, small dust particles, very large molecules)
1980s 90s: Submillimeter astronomy (warm interfaces of molecular clouds, cold protostellar regions)
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The Impact of Space Astronomy
1973 80: Copernicus UV satellite
Very highly ionized atoms (e.g., O VI) very hot , tenuous component of ISM
Leads to 3-phase model of ISM
1983: IRAS: Full-sky survey at 12, 25, 60 and 100 m
NL participation
Presence of very small dust particles (10-100 ) and/or large molecules (PAHs?)
Ultra-luminous infrared galaxies
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IRAS All-Sky Image
Blue: 12 m Green: 60 m Red: 100m
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The Milky Way is largely empty
distance between stars 2 pc
heliosphere 235 AU
stars occupy 3x10-10 fraction of MW volume
The remaining 0.9999999997 filled by the ISM
hydrogen, helium, + traces of metals
ionized (H II, ...), neutral (H I, ...), molecular (H2, ...)
gas phase or solid state (dust, ice, ...)
What is the ISM?
(Tielens, Ch 1)
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ElementAbundance
(by number)Element
Abundance
(by number)
H 1 Mg 4.3710-5
He 0.095 Al 2.9510-6
C 2.9510-4 Si 3.5510-5
N 7.4110-5 S 1.4510-5
O 5.3710-4 Ca 2.1410-6
Na 2.0410-6 Fe 3.4710-5
Protosolar abundances (Asplund 2009)
based on photospheric and meteoritic measurements;
ISM abundances in solar neighbourhood thought to be similar
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Classical approach: Observationally distinct objects
H II regions
reflection nebulae
dark clouds
supernova remnants
molecular clouds
More physical classification in different phases:
cool molecular clouds
cool H I clouds
warm intercloud gas
hot coronal gas
What are the basic properties of the phases and how are they related?
The Galactic ecosystem
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Objects: HII Regions
H II regions surrounding early-type (25,000 K)
stars, emitting lots of photons beyond Lyman limit (13.6
eV)
ionized gas, bright visible nebulous objects
Associated with massive star-forming regions
optical spectra dominated by H and He recombination lines;
collisionally excited, (forbidden) optical lines from ions
like [O II], [O III], and [N II]
strong sources of thermal radio emission (free-free) +
infrared emission from warm dust
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Objects: Reflection Nebulae
Nebulae reflecting light from nearby stars
e.g., NGC 2023 in Orion; emission around the
Pleiades
No radio emission, but infrared emission from
warm dust present
Illuminated by stars later than B1 (no ionizing
radation)
Either cloud material from which star was
formed; or chance encounter (Pleiades!);
sometimes ejecta of late-type stars (e.g., Red
Rectangle)
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Objects: Dark nebulae
Dark bands straddle the Milky Way
Dark clouds range from tiny (0.01 pc) so-
called Bok globules, to tens of pc for
large clouds; large range in AV
Sometimes very faint reflected light +
often bright in mid- and far IR
Some even dark at mid-IR : Infrared Dark
Clouds (IRDCs)
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Objects: Supernova remnants (SNR)
Left over ejecta from SN explosion
About 100 SNRs visible in MW
Filamentary and shell-like structures (but some
compact, e.g., Crab), emitting line radiation
Strong in radio due to synchrotron emission;
and bright in X-rays because of hot (106 K gas)
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Phase: Neutral atomic gas
Traced by H I 21 cm line or optical/UV
absorption lines of a variety of elements
against background stars
Consists of
cold, diffuse H I clouds (100 K): CNM
warm intercloud gas (5000 K): WNM
Galactic distribution
its everywhere!
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Phase: Ionized gas
traced through H emission, optical/UV ionic absorption lines,
pulsar dispersion
H emission dominated by H II regions, but most ionized gas
resides in a huge, diffuse reservoir (109 M)
Warm Ionized Medium (WIM), 0.2 cm-3, 8000 K
Ionized by photons escaped from HII regions
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Phase: Molecular gas
Traced through CO lines (2nd most abundant molecule,
CO/H2104) at millimeter wavelengths, since H2 difficult to
observe
Concentrated in Giant Molecular Clouds
40 pc, 4x105 M, 200 cm-3, 10 K
But: large range in properties, and complex substructure
Self-gravitating
Pressure from turbulence and magnetic fields important
Sites of Star Formation
200+ molecular species detected
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Phase: Coronal gas
Coronal gas: very hot, tenuous gas pervading the ISM
T ~ 105.5 106 K
n 0.004 cm-3
traced through highly ionized species, C IV, S VI, N V, O VI
in absorption against background stars; also: free-free
emission, radiative recombination, UV, X-ray lines
Fills most of the halo; disk less clear
heated, ionized by (SN) shocks; Sun in Local Bubble;
Galactic fountain fills the MW halo
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Example: FUSE Spectrum of CSPN K1-16
Absorption lines due to intervening gas marked below
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Overview ISM phases in Milky Way
phase n (cm-3) T (K) fV
coronal (HIM) ~0.004 >105.5 ~0.5?
warm, neutral
(WNM)~0.6 ~5000 ~0.4
warm, ionized
(WIM)~0.2 ~8000 ~0.1
cold, neutral
(CNM)~30 ~100 ~0.01
molecular
clouds~103-6 ~10-50 ~0.0001
HII regions ~1-105 ~104 very small
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Cycle of material between phases
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Added ingredient: Interstellar dust
absorption, scattering, reddening, extinction, polarization, infrared
emission
~1% of gas mass
Much C, Si, Mg, Fe, Al, Ti, Ca (=refractory elements) locked up in dust:
depletion
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ISM mass budget
MW: 1.8x1011 M stars; 6.7x109 M gas
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Energy sources and densities
Radiation
Magnetic fields
Cosmic rays
Mechanical energy
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Energy Densities in Local ISM
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Energy densities
All six energy densities are of comparable magnitude
uthermal , uhydro , umagnetic are coupled (magneto-)
hydrodynamically
uthermal is (weakly) coupled to ustarlight
u3K CBR is not coupled to anything else
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In thermodynamic equilibrium at temperature T, the
Maxwell, Boltzmann, and Planck distributions apply
Maxwell distribution of velocities
Boltzmann distribution of population of energy levels
gu and gl are statistical weights of upper and lower levels
ISM: basic physical conditions
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Thermal equilibrium requires detailed balance, i.e., each process
occurs as often as the inverse process
This is frequently not true in ISM, e.g., collisional excitation is
followed by radiative decay (because of low density)
Example: O2+=O III in H II region
Collisions
5007
4363 2321
1S
3P
1D
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Physics and chemistry: out of thermal
equilibrium
Energy flow between states, phases
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Maxwell: yes
Elastic collisions are sufficiently frequent to thermalize velocity distribution
Usually Tkin Te = Ti = Tn (NB: exceptions exist)
Boltzmann: no
Define excitation temperature Tex by
In general Tex Tkin
Which Distributions are Valid in ISM?
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Statistical Equilibrium
Because radiation field cannot be described by Planck function, thermal equilibrium does not hold if both radiative processes and collisions are important
Proceed by assuming statistical equilibrium:
Sum of rates of all processes populating level i = sum of rates of all processes depopulating level i
Q: what is difference with detailed balance?
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Next lecture: Review of Radiative Processes
Radiation definitions and quantities
Einstein coefficients
Radiative transfer